This invention relates to a loudspeaker system for use in a stereophonic reproduction system.
For a stereophonic reproduction, in general, a pair of speaker devices SL and SR are placed at equal distances from a listener P as shown in FIG. 1 and radiate stereophonic acoustic energies in response to application of left and right stereophonic signals L and R thereto.
It is usually said that for a continuous sound a human being senses the direction of a sound source by the sound pressure difference (mainly high frequencies) and phase difference (mainly low frequencies) of sounds reaching the right and left ears of a listener and that, for a pulsive sound, i.e., an instantaneous sound, the direction of the sound is sensed primarily dependent upon the time difference and amplitude difference between sounds reaching the listener's right and left ears. It is considered that, for a sound source located between left and right channels (phantom channel), the sound pressure difference and phase difference are provided between left and right channel signals L and R according to the direction of the sound source, and that the sense of direction of the sound source, i.e., the sense of location of a reproduced sound image for the phantom channel can be obtained by auditorily sensing the pressure difference, and phase difference of sounds radiated from the right and left speaker devices SR and SL. That is, it is believed that for a phantom sound source located at the front center, for example, a reproduced sound image is located at the front center provided that the left and right speaker systems simultaneously radiate acoustic energies which are same in magnitude and phase. However, a component Ll of a left channel signal L from the left speaker device SL and a component Rl of a right channel signal R from the right speaker device SR are applied to the left ear El of the listener P. Likewise, a component Rr of the right channel signal R from the right speaker device SR and a component Lr from the left speaker device SL are applied to right ear Er of the listener P. As will be evident from FIG. 1, distances SL-Er and SR-El are greater than distances SL-El and SR-Er in dependance upon the positional relation of the left and right speaker systems SL and SR. Accordingly, the left channel component Lr to the listener's right ear Er and right channel component Rl to the listener's left ear El are somewhat time-delayed with respect to the left channel component Ll to the listener's left ear El and right channel component Rr to the listener's right ear Er. The presense of such left and right channel components Rl and Lr offers a greater bar to the listener's sense of direction to a phantom sound image, imparing the location of the phantom sound image. This provides one decisive cause for an auditory difference between a real sound source and a phantom sound source. The sound image location is deteriorated due to the signal components El and Er other than the channel signals Ll and Rr corresponding to the listener's left and right ears El and Er, and such a deterioration prominently appears for a pulsive signal in particular due to the time delay as set out above. Where a pulsive signal to be located, for example, at the front center in the reproduction sound field as shown in FIG. 1 is provided, responses at the left and right ears El and Er of the listener P are as shown in FIGS. 2A and 2B. Left and right channel signals L and R are radiated, in the same phase and amplitude, simultaneously from the left and right speaker devices SL and SR and applied as left and right channel components Ll and Rr to the left and right ears El and Er, respectively. After lapse of a time .DELTA.t, leftand right channel components Lr and Rl are applied to the right and left ears Er and El of the listener P, respectively. When, however, a pulsive sound is provided from an real sound source located at the front center, responses at the listener's left and right ears El and Er are only such that components Ll' and Rr' are applied in the same phase and amplitude to the left and right ears El and Er of the listener, respectively, as shown in FIGS. 3A and 3B. Since the components Ll and Rr are similar to the components Ll' and Rr' respectively, a difference in response between the phantom sound image and the actual sound image resides in that for the phantom sound source the components Rl and Lr are applied with a time delay .DELTA.t after the left and right channel components Ll and Rr are applied to the listener's left and right ears El and Er. Such a time delay is usually of the order of 200 .mu.sec. It is said that a time difference with which the human being senses two pulsive signals as one signal through his ears is of the order of below about 60 .mu.sec. Such time difference .DELTA.t is to the extent of being sufficiently sensed by the ears of the human being, the responses shown in FIGS. 2A and 2B are substantially the same as in the case where similar sound sources are positioned in the respective positions of the left and right speaker devices SL and SR, or two real sound sources are positioned at the center front at different positions from the listener P. For the pulsive sound, therefore, the localization of the sound image becomes indefinite.
The components Lr and Rl have a time difference and phase difference with respect to the inherent left and right channel components Ll and Rr, making the rise time and phase relation of the inherent left and right channel signals indefinite and preventing the sensing of the direction of a sound image in dependence on the time difference and phase difference between the left and right channel signals.
Since musical sounds are comprised of a combination of continuous sounds and pulsive sounds, unless the deterioration of such a sound image location characteristic is prevented as far as possible, it is impossible to attain a good sound image reproduction.
In view of a recent reduction to practice of four-channel stereophonic systems it is desired to extend the location range of the sound image in a two-channel stereophonic reproduction system using a pair of speaker devices.